This invention relates generally to radiation imaging systems and more particularly to scintillator designs to improve light collection efficiency.
Radiation imaging systems are widely used for medical and industrial purposes. For example, in nuclear medicine, certain diagnostic tests involve injection into the patient of radionuclides which will concentrate in an organ of interest. Radiation emitted from these radionuclides can be used for examining the organ's structure and its operation. External radiation sources, such as x-ray machines, are also be used for diagnostic purposes. Imaging systems have been developed which used the detected radiation to produce a signal which can be used to operate a visual display, such as a cathode ray tube or liquid crystal display device, or which can be used for other analyses of the pattern of detected x-ray or gamma radiation. In such systems the radiation is typically absorbed in a scintillator material, resulting in generation of photons of light. Light photons emanating from the scintillator are detected by photodetectors to generate an electrical output signal that can be processed to drive the display or analysis system.
Particularly for radiation imagers employed in nuclear medicine procedures, in which it is desired to map the emitted radiation of a very low energy radionuclide taken up in the tissue of the patient, it is important that the imaging device be sensitive to low radiation levels while still being able to discriminate against background radiation. Efficient collection of the light photons generated when the incident radiation is absorbed in the scintillator allows detection of radiation with lower energy levels and thus enhances the diagnostic value of the imaging device.
Light photons generated when incident radiation is absorbed in scintillator material propagate isotropically from the place of the absorption of the incident radiation. Most scintillator structures have a parallelepiped shape, with the photodetectors adjoining one side of the scintillator. Consequently, only a limited number of the photons generated will propagate directly toward the scintillator surface adjoining the photodetector, and consequently it is beneficial to have some means for reflecting and directing other photons towards that surface. Typical prior art imagers provided no reflective material on the scintillator surfaces, such as evidenced by the device of Beerlage disclosed in U.S. Pat. No. 4,906,850. In some devices, it has been suggested that optically opaque materials can be applied to the sidewalls of the scintillator elements, as appears in the device of Iverson disclosed in U.S. Pat. No. 3,936,645 (see col. 7, line 46), or that the scintillator surfaces can be made reflective by polishing or metallizing, as disclosed in U.S. Pat. No. 3,507,734 of Ruderman (see col. 3 line 62-66).
In scintillators having reflective wall surfaces, the reflected photons typically travel a path having numerous interactions with the scintillator wall surfaces before they strike the surface of the scintillator adjoining the photodetector array. Particularly with scintillator geometries having opposed parallel smooth surfaces, a number of photons will undergo total internal reflection, that is, a photon will strike the scintillator surface adjoining a photodetector at an angle that will cause it to reflect back into the scintillator instead of exiting the scintillator, and then travel a path reflecting off other surfaces of the scintillator that will keep it within the scintillator, or involve sufficient interactions with the wall surfaces or reflectors so that the probability that the photon will be absorbed, and hence undetected by the photodetector array, is significantly increased. In either case, total internal reflection results in a smaller number of the total number of photons generated by the absorption of incident radiation in the scintillator from exiting the scintillator for detection by the imager array, thus reducing the photon collection efficiency of the device.
Light photon attenuation remains a problem even when an optically reflective material has been applied along the sidewalls of the scintillator. In optically reflective materials there is nevertheless some absorption of the light, and thus light photons may be either absorbed or reflected when they strike the optically reflective material on the walls of the scintillator. For example, silver has a reflectance of 96%; with that reflectance, a light photon has a 50% chance of being absorbed after just 17 wall interactions. Other commonly used optically reflective materials have even lower reflectance values, such as the 90% value for aluminum. Imaging device performance is thus degraded when a light photon undergoes numerous reflections because there is an increased likelihood that the light photon will be absorbed. Additionally, when those photons that do escape the scintillator to the photodetector are detected over a longer period of time (i.e., some photons generated will exit the scintillator quickly while others will be reflected numerous times before exiting the scintillator) there is not as clear a "peak" of a photon burst to be detected by the photodetectors; this lack of a peak degrades the energy resolution of the imaging device and makes it more difficult for the processing system to discriminate against background noise.
It is thus an object of the present invention to provide an imaging device that exhibits a high light photon collection efficiency.
Another object of the invention is to provide an efficient scintillator that exhibits minimal total internal reflection.
It is a further object of this invention to provide a simple and effective means of reflecting and focussing light photons generated in the scintillator towards the scintillator/photodetector interface.